Attach dev.cpu nodes on powerpc SMT cores, using only the first found thread

Summary: In order to use cpufreq(4), a dev.cpu attachment must be created. If
the IBM property is found denoting SMT, attach only to the first thread setup,
so that a cpufreq device can bind.

Reviewed by: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D15921